Electrical double layer capacitors (EDLCs), also called supercapacitors or ultracapacitors, have received a lot of interest lately due to their potential for providing high power densities. However, they have fallen short in energy-density capabilities, which has curtailed their widespread application as an alternative, more powerful energy source to conventional batteries. Commercially available supercapacitors today are constructed from activated carbon electrodes made primarily from coconut-shell charcoal powder and have surface areas of 2000 m2/g and energy densities of ˜6 Wh/Kg (Pandolfo, A. G. et al., J. Power Sources, 2006, 157:11-27; Conway, B. E. et al., Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, 1999, Kluwer Academic/Plenum, New York; Gamby, J. et al., J. Power Sources, 2001, 101(1):109-116; Burke, A., J. Power Sources, 2000, 91:37-50; “Basic Research Needs for Electrical Energy Storage”, Report of the Basic Energy Sciences Workshop on Electrical Energy Storage, Office of Basic Energy Sciences, DOE, July 2007). Capacitance in these devices is determined by the surface area, pore size and its distribution, nature and concentration of the surface functional groups of the carbon material used to construct the electrodes, and the electrolytes (aqueous, organic or ionic solvents).
EDLC electrodes are typically constructed by mixing coconut-shell carbon powder with various binders and additives (up to 15%) to improve the mechanical and electrical properties, and then rolling and compacting the powder into sheets. Multiple sheets (with a separator to electrically isolate adjacent sheets) are then packaged, cut to size and filled with electrolyte to form the supercapacitor (example in FIG. 1). Different electrolytes like aqueous solutions (e.g. H2SO4, KOH), organic solutions (acetonitrile, propylene carbonate) or ionic liquids can be used to provide different energy density and voltage characteristics, although only organic-solvents-based devices have achieved any commercial success so far.
The key features that enable higher specific-capacitance from electrodes are the surface chemistry and the nature of the porosity of the electrodes. Several attempts are underway to fabricate electrodes using novel processing techniques that include the use of “Hard” and “Soft” templates. “Hard templating” is defined as a process in which the template and the carbon sources are made separately. The interconnecting pore structure of the template is made before the templating process. The pores are then filled with the carbon source and the template is removed after the formation of the carbon matrix. “Soft templating” is defined as a process in which the template and the carbon source are synthesized as a composite material and the template is formed as an embedded network of non-carbon material within the carbon matrix. The soft template is then removed to make the porous carbon.
Current industry efforts to make novel EDLC electrodes fall into the following three categories:                1) “Hard” templates into which the electrode materials are deposited; examples include the use of long-chain organic surfactant templates described by Nanotecture (U.S. Published Application No. 2009/0170000; Coowar) and Si-oxide templates disclosed by Nanotune (U.S. Pat. No. 8,454,918; Wang et al.);        2) “Soft” templates of non-carbon elements embedded within a carbon matrix: examples include techniques disclosed by Skelton Technologies (U.S. Pat. No. 7,803,345; Leis et al.) and YCarbon (U.S. Published Application No. 2012/0148473; Kramarenko);        3) Techniques to grow electrode surfaces (primarily in monolithic form) using exotic materials like graphene, carbon nano tubes (CNT), etc.; examples include U.S. Published Application No. 2010/0035093 (Ruoff et al.) and U.S. Published Application No. 2012/0134071 (Sadoway et al.).        